The “going green” movement is one that has picked up tremendous momentum in the United States. From energy and water conservation, to recycling, to buying organic foods and even clothing, people are more earth-conscious than ever. One (1) of the most popular means of going green involves reducing fossil fuel consumption. As available supplies dwindle, it is apparent that there is a need for a system to converting carbon dioxide from heat and power generators into synthetic gas without producing greenhouse gas emissions during the transformation.
When petroleum and natural gas were very expensive, it is a necessity to use biomass, coal and other carbonaceous source as fuel. Coal gasification processes are reasonably efficient and were used for many years to manufacture illuminating gas (coal gas) for gas lighting.
Gasification comprises burning of a feedstock in a reactor at a temperature in the range of eight hundred to fifteen hundred degrees Celsius (800-1500° C.) in the presence of air, or oxygen and water. Synthesis gas is obtained by reaction between carbon dioxide, which produced by combustion of feedstock to more than five hundred fifty degrees Celsius (550° C.) carbonaceous substances. Synthetic gas has a heating value from 10500 to 14600-16700 kJ/m3 (under normal conditions). This gas is a mixture of carbon monoxide and hydrogen; mixtures of methane with other hydrocarbons are possible. Like direct combustion, gasification is a high-temperature thermochemical conversion process, but the desired result in this case is the production of a combustible gas instead of heat. This is achieved through the partial combustion of the feedstock in restricted supply of air or oxygen, usually in a high temperature environment. The product of gasification—synthetic gas—can, after appropriate treatment, be burned directly for cooking or heat supply, or it can be used in secondary conversion technologies such as gas turbines and engines for producing electricity or mechanic work.
Synthetic gas is the name given to gases of varying composition that are generated the gasification reactor or some types of waste-to-energy gasification facilities. Synthetic gas is also used as an intermediate in producing synthetic petroleum for use as a fuel or lubricant via Fischer-Tropsch synthesis.
Synthetic gas consists primarily of carbon monoxide and hydrogen, and has less than half the energy density of natural gas. Synthetic gas is combustible and often used as a fuel source or as an intermediate in the production of other chemicals. Synthetic gas for use as a fuel is most often produced by gasification of coal or municipal waste. As an intermediate in the large-scale, industrial synthesis of hydrogen and ammonia, it is also produced from natural gas. The synthetic gas produced in large waste-to-energy gasification facilities is used as fuel to generate electricity.
Gasification is a thermochemical process that generates a gaseous, fuel rich product. Regardless of how the gasification reactor is designed, two (2) processes must take place in order to produce a useable fuel gas. In the first stage, pyrolysis releases the volatile components of the fuel at temperatures below six hundred degrees Celsius (600° C.) (1112° F.). The by-product of pyrolysis that is not vaporized is called char and comprises mainly of fixed carbon and ash. In the second gasification stage, the carbon remaining after pyrolysis is either reacted with steam or hydrogen or combusted with air or pure oxygen. Gasification with air results in a nitrogen-rich, low BTU-fuel gas. Gasification with pure oxygen results in a higher quality mixture of carbon monoxide and hydrogen and virtually no nitrogen. Gasification with steam is more commonly called “reforming” and results in a hydrogen and carbon dioxide rich “synthetic” gas. Typically, the exothermic reaction between carbon and oxygen provides the heat energy required to drive the pyrolysis and char gasification reactions.
The basic gasification reactions that must be considered are:C+H2OCO+H2 C+CO22COCH4+H2OCO+3H2 CH4CO22CO+2H2 
All of these reactions are reversible and their rates depend on the temperature, pressure and a concentration of oxygen in the gasification reactor.
If carbon dioxide (CO2) passes a layer of feedstock by five hundred fifty degrees Celsius (550° C.) and higher, CO2 converts into carbon monoxide (CO). By this reaction (CO2+C═CO+CO) from two (2) molar volumes of carbon dioxide make four (4) molar volumes of carbon monoxide and, on the contrary, when carbon monoxide combusts in the reaction with oxidant from two (2) molar volumes of carbon monoxide makes one (1) molar volumes of carbon dioxide.
The basis of the formation of synthetic gas is a process that air is introduced to a lower layer of heated carbonaceous feedstock and creates carbon dioxide CO2 and produces heat adequate for heating the feedstock and CO2. Subsequently, by interaction in upper layers without oxygen the carbon dioxide and heat produces carbon monoxide. The reaction moved forward due to absorption of heat.
The design and operating parameters of the gasification reactor promise low level particulate emissions. Feed stocks containing up to fifty-five percent (55%) moisture have been successfully converted to clean hot gas. The low particulate emission plus the generally lower inorganic content of biomass fuels translates into reduced emission of particulate air toxic materials. Due to the precise control of the gasification and combustion zone conditions and temperatures, pollutant by-products of combustion reactions such as NOx emissions may be lower than in conventional boilers even when fuel with a higher fixed nitrogen are used. The air intake is at the bottom and the synthetic gas leaves at the top. Near the grate at the bottom the combustion reaction occurs, and the synthetic gas ins produced by reduction somewhat higher up in the gasification reactor. In the upper part of a gasification reactor, heating and pyrolysis of the feedstock occurs as a result of heat transfer by convention and radiation from the lower zones. The tars and volatiles produced during this process will be carried in the gas stream. Ashes are removed from the bottom of the reactor.
The product gases from gasification can be used for energy production, fuels, or chemical production. A separate combustion chamber outside the gasification chambers is often used for energy production. The thermal energy resulting from the combustion of gaseous products can be used in a variety of ways. These include the production of steam for generating electricity and thermal energy for the production of heat, which can then be used to bolster the reaction within the gasification reactor.
An important component of any gasification combustion process is the after-treatment equipment used to clean the effluent gases. Although gaseous products can typically be combusted more efficiently than solid materials, advanced emission control systems would still be required to meet regulatory standards. Typical exhaust or flue gas control strategies for combustion processes include particulate filters or bag houses, wet scrubber techniques, or electrostatic precipitators. The post-processing of solid like char, and ash from gasification, is another important process step. Similarly, the char, or solid carbonaceous portion of the residue, can either be utilized as a fuel for the process or sold as a carbon-rich material for the manufacture of activated carbon or for other similar industrial purposes. The reintroduction or use of char as a fuel source in the process is an important element in the process design for many of the technologies surveyed. The inert ash in the gasification residual is generally not reintroduced into the process; however, the ash may be incorporated in many technologies. This could include water wash/quenching, screening, and the removal of metals. In some technologies, a vitrification step is also included whereby the ash is heated to a temperature above the fusion point of sand, which can then incorporate the soluble components of the ash to produce an impervious residual slag that can inhibit leaching of the ash components into ground water when buried.
The basic sources of carbon dioxide are power and heat generators: engine, turbine, and other equipments. A typical coal plant has an efficiency in the low thirty percent (30%) range, meaning sixty-five (65%) or more of the energy is wasted. Seventy percent (70%) of the nation's energy is rejected to the atmosphere as waste energy. More than forty percent (40%) of energy rejected to atmosphere with exhaust gas from mobile generators. It is often difficult to find useful application for large quantities of heat, so the heat is qualified as waste heat and is rejected to the environment. Economically most convenient is the applying of such heat to a gasification process; it is a huge resource of energy, which can be used for converting carbon dioxide into synthetic gas. The results of operation for utilizing waste heat in order to improve the efficiency and to heat feedstocks on the basis of environmentally friendly technologies are considered.